Fix incorrect argument to Checker Framework, and fix nullness warnings. (#1354)

709d97aa321d5729988fd63b960bbece04cfba10 modified the -AskipDefs argument to the
Checker Framework (a regular expression) in a way that caused it to skip
checking all files.  This commit fixes the regular expression and the new
Checker Framework warnings.
diff --git a/build.gradle b/build.gradle
index f021d25..e76d945 100644
--- a/build.gradle
+++ b/build.gradle
@@ -410,7 +410,7 @@
                     compile.options.compilerArgs += [
                         '-Xmaxerrs', '10000',
                         "-Xbootclasspath/p:${configurations.checkerFrameworkAnnotatedJDK.asPath}",
-                        "-AskipDefs=[\\.AutoValue_|\\.TraceIdProto]",
+                        "-AskipDefs=\\.AutoValue_|^io.opencensus.contrib.appengine.standard.util.TraceIdProto\$|^io.opencensus.contrib.appengine.standard.util.TraceProto\$",
                         "-AinvariantArrays"
                     ]
                     options.fork = true